PROJECT SUMMARY New drugs have transformed Multiple Myeloma (MM) from a terminal blood cancer, affecting approximately 83,000 people in the U.S., to a disease responsive to targeted therapies. Among the most effective MM treatments are the immunomodulatory (IMiD) cereblon-binding drugs (e.g. lenalidomide, pomalidomide). These IMiDs inhibit the critical MM cell pro-survival interferon regulatory factor 4/myelocytomatosis viral oncogene (IRF4/MYC) pathway indirectly by promoting the cereblon (CRBN) mediated degradation of the IRF4 transcriptional regulator, Ikaros (IKZF1). Although most patients initially respond to IMiD therapy, resistance is inevitable. One mechanism involves a decrease of CRBN activity, which appears to be due to genetic alterations or modified expression of CRBN and IKZF1. The second mechanism involves upregulation of CD44, the main cell surface receptor for hyaluronan, involved in MM cell adhesion. Thus far, no approved agent used in MM is known to modulate CD44 and IRF4/MYC, the key mediators of IMiD resistance. Drug resistance in MM also seems to involve epigenetic modifications, affecting expression of small non-coding RNAs (miRNAs, miRs) and drug transporters. our preclinical studies revealed that AR-42, a pan-histone deacetylase inhibitor, downregulates the IRF4/MYC pathway and CD44 expression via miRNAs upregulation and synergistically enhances IMiD anti-MM activity. Our first in human, single agent trial of AR-42 in relapsed MM demonstrated the agent was safe with some prolonged clinical responses. Our central hypothesis is that AR-42 sensitizes MM cells to IMiD treatment by upregulating miRNAs that reduce CD44 expression (and thus drug efflux) and inhibit the IRF4/MYC pathway. To further test our central hypothesis, it is important to determine if CD44 is playing a functional role in IMiD resistance and if miRNA regulation can bypass genetic alterations in IMiD targets associated with resistance in order to: (a) understand a functional role of a surface protein that can be therapeutically targeted and/or used as a marker of IMiD resistance; and (b) establish if miRNAs have the potential to be used therapeutically to overcome mechanisms of IMiD resistance. Our team will pursue the following specific aims: 1) Investigate the role of CD44 in MM IMiD resistance in vitro and in vivo; 2) Investigate CD44 and IRF4/MYC regulation via miRs with IMiD therapy; and 3) Conduct a phase 1b study of AR-42+Pomalidomide in Lenalidomide-resistant MM patients. Outcomes: Our studies will define the relationship of CD44 expression, intracellular IMiDs levels, and IMiD resistance in MM. We will understand whether sCD44 level in the serum of MM patients is a valuable non invasive marker for an early clinical assessment of the development of IMiD resistance that could guide clinicians to change therapy before relapse is occurring. The improved understanding concerning miRNA-modulated IMiD resistance will support future clinical studies aimed at optimizing the efficacy and duration of response to IMiDs in MM and a variety of other cancers.